Polyesters, especially polyalkylene terephthalates, have excellent physical and chemical properties and have been widely used for resins, films and fibers. In particular, polyester fibers have relatively high melting points, and can attain high orientation and crystallinity. Accordingly, polyesters have excellent fiber properties such as chemical, heat, and light stability, and high strength.
However, polyesters such as poly(propylene terephthalate) have relatively low toughness or impact strength when compared with other polymers such as poly(oxymethylene) (e.g., DELRIN available from E.I. du Pont de Nemours & Co., Wilmington Del.; hereinafter all terms written in upper cases are wither trade names or trademarks), nylon 66 (e.g., ZYTEL from the same source), and poly(butyleneterephalate-co-polytetramethylene glycol terephthalate) (e.g., HYTREL from the same source). Toughness is conventionally measured on an Instron machine (a tensile method, with units of in-lbf/in.sup.3 or J/m.sup.3). Impact strength is conventionally measured by the "Izod" test wherein a heavy pendulum swings against a standard notched bar of the polymer. The energy required to just break the bar is recorded in ft-lbf/in or in-lbf/in (J/m, see Test Methods below). Poly(propylene terephthalate) impact strength measured by the Izod test is about 0.5 ft-lbf/in (27 J/m). By comparison, other polymers such as poly(oxymethylene), nylon 66, and poly(butylenelpoly tetramethylene ether terephthalate) have impact strengths of about 1.3, 2, and 3.7 ft-lbf/in (69, 107, and 198 J/m) respectively.
In other applications of poly(propylene terephthalate), the polymer is found to be difficult to dye. The high levels of orientation and crystallinity that impart desirable properties to the polyester contribute to the difficulty in dyeing, but a major factor is that polyesters, unlike protein fibers, do not have ionic sites within the polymer chain that are reactive to basic or acid dye compounds. A number of comonomers can be copolymerized with the polyester or polyamide as a means of conferring basic dyeability. Notable examples are aromatic sulfonates, their sodium salts, and specifically the sodium salts of 5-sulfoisophthalic acid or of dimethyl 5-sulfoisophthalate. While the use of comonomers such as 5-sulfoisophthalate salts provide an effective site for cationic dyes, the comonomers do not improve impact strength.
Elsewhere, the introduction of "soft segments" into polymer chains has been found to improve impact strength. Examples of soft segments are sections of the polymer chain comprised of poly(oxyalkylene) units. However, the extent to which impact strength may be increased by the introduction of such soft segments is limited by concurrent reduction in the tensile strength.
WO099/09238 discloses a polyester fiber prepared from a polyester prepared by copolymerizing polytrimethylene terephthalate with a third component. The third component is an ester-forming sulfonate compound used in a comononmer ratio of 0.5 to 5% by mole.
U.S. Pat. No. 5,097,004 discloses polyesters based upon polyethylene terephthalate copolyrnerized with a polyethylene glycol and a 5-sulfoisophthalic acid and, if desired, a polyethylene ether such as diethylene glycol.
There remains a need to develop a composition and a process for improving further the impact strength of the polyester composition.